The biologic activities of the alternate complement pathway will be investigated in terms of the physiologic triggering agents, antigen- antibody complexes. Modulation of this complement function will be studied with activity assays for the early-acting components in the sequence (properdin, Factor D and C 3b) as well as for Factor B, the C3 proactivator. The in vitro and in vivo utilization of these reactants will be evaluated as a function of the immunoglobulin class and the ratio of antigen and antibody in the complex. The role of the alternate complement pathway in the induction of antibody formation will be examined. Studies are also planned to characterize the nature of the immune event and changes in alternate pathway components which are associated with elevated activities in the sera of animals with neoplastic disease and in pregnancy.